The catalytic epoxidation of olefins over silver-based catalysts, yielding the corresponding olefin oxide, has been known for a long time. Conventional silver-based catalysts have provided the olefin oxides with notoriously low selectivity. For example, when using conventional catalysts in the epoxidation of ethylene, the selectivity towards ethylene oxide, expressed as a fraction of the ethylene converted, does not reach values above the 6/7 or 85.7 mole-% limit. Therefore, this limit has long been considered to be the theoretically maximal selectivity of this reaction, based on the stoichiometry of the reaction equation7C2H4+6O2=>6C2H4O+2CO2+2H2O,cf. Kirk-Othmer's Encyclopedia of Chemical Technology, 3rd ed., Vol. 9, 1980, p. 445.
The selectivity determines to a large extent the economical attractiveness of an epoxidation process. For example, one percent improvement in the selectivity of the epoxidation process can reduce the yearly operating costs of a large scale ethylene oxide plant substantially.
The olefin oxide produced by the epoxidation process may be reacted with water, an alcohol or an amine to form a 1,2-diol, a 1,2-diol ether or an alkanolamine. Thus, 1,2-diols, 1,2-diol ethers and alkanolamines may be produced in a multi-step process comprising olefin epoxidation and converting the formed olefin oxide with water, an alcohol or an amine. Any improvement in the selectivity of the epoxidation process can also reduce the yearly operating costs in the overall process for the production of a 1,2-diol, a 1,2-diol ether or an alkanolamine.
Modern silver-based epoxidation catalysts are highly selective towards olefin oxide production. When using the modern catalysts in the epoxidation of ethylene the selectivity towards ethylene oxide can reach values above the 6/7 or 85.7 mole-% limit referred-to. Such highly selective catalysts comprise, in addition to silver, a selectivity enhancing dopant which may be selected from rhenium, molybdenum, tungsten and nitrate- or nitrite-forming compounds, cf. for example U.S. Pat. No. 4,761,394 and U.S. Pat No. 4,766,105.
A reaction modifier, for example an organic halide, may be added to the feed to an epoxidation process for increasing the selectivity (cf. for example EP-A-352850, U.S. Pat. No. 4,761,394 and U.S. Pat. No. 4,766,105, which are herein incorporated by reference). The reaction modifier suppresses the undesirable oxidation of olefin or olefin oxide to carbon dioxide and water, relative to the desired formation of olefin oxide, by a so-far unexplained mechanism. EP-A-352850 teaches that there is an optimum in the selectivity as a function of the concentration of organic halide in the feed, at a constant oxygen conversion level and given set of reaction conditions.
During the initial phase of an epoxidation process, the catalyst experiences the so-called “break-through phase” during which the oxygen conversion is very high, the selectivity is very low, even in the presence of a reaction modifier, and the epoxidation process is difficult to control. It might take a long time in the start-up of a commercial epoxidation process for the conversion to drop so that the reaction can more easily be controlled at an attractive level of the selectivity. It goes without saying that there is an economical incentive to shorten the start-up period and make the catalyst operate at a high selectivity with a minimum delay.
U.S. Pat. No. 5,155,242 relates to the start-up of an epoxidation process wherein a conventional catalyst is employed. In this patent document there is disclosed an improved start-up procedure wherein the conventional catalyst is subjected to a pre-soak period in the presence of the organic halide at a temperature less than the operating temperature of the reactor.
U.S. Pat. No. 4,874,879 relates to the start-up of an epoxidation process wherein a highly selective catalyst is employed. In this patent document there is disclosed an improved start-up procedure wherein the highly selective catalyst is subjected to a pre-soak period in the presence of the organic halide at a temperature less than the operating temperature of the reactor. This procedure may alleviate to some extent the problems associated with the duration of the start-up. However, it has been experienced that, still, it takes several days for the catalyst to pass through the break-through phase. This results in considerable losses of olefin oxide production, as described hereinbefore.
WO-95/05896 proposes a silver-based catalyst which comprises, as a further component, a selected quantity of chloride. Such catalysts have improved start-up characteristics over catalysts which do not comprise chloride.